Zinc electrodes for batteries

ABSTRACT

An article having a continuous network of zinc and a continuous network of void space interpenetrating the zinc network. The zinc network is a fused, monolithic structure. A method of: providing an emulsion having a zinc powder and a liquid phase; drying the emulsion to form a sponge; annealing and/or sintering the sponge to form an annealed and/or sintered sponge; heating the annealed and/or sintered sponge in an oxidizing atmosphere to form an oxidized sponge having zinc oxide on the surface of the oxidized sponge; and electrochemically reducing the zinc oxide to form a zinc metal sponge.

This application is a continuation-in-part application of U.S.application Ser. No. 13/832,576, filed on Mar. 15, 2013, which claimsthe benefit of U.S. Provisional Application No. 61/730,946, filed onNov. 28, 2012. The provisional application and all other publicationsand patent documents referred to throughout this nonprovisionalapplication are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure is generally related to porous zinc electrodesfor use in batteries and other uses.

DESCRIPTION OF RELATED ART

The ongoing effort to fulfill the exigencies of ever-growing energymarkets, including electric vehicles and portable electronic devices,has led to the investigation of battery technologies that promise toovercome some of the pitfalls of Li-ion batteries. While Li-ionbatteries have the benefits of low self-discharge, no memory effect, andabove all, rechargeability, the broader application of Li-ion-basedenergy storage is limited by safety concerns, manufacturing costs, andlower specific energy densities (<200 W h kg⁻¹) relative to otherpromising battery technologies (Lee et al., “Metal-air batteries withhigh energy density: Li-air versus Zn-air” Adv. Energy Mater. 2011, 1,34-50). Zinc-air batteries, for example, have high practical specificenergy densities (400 W h kg⁻¹) and the advantage of a cheap andenvironmentally friendly active material (zinc) coupled to air-breathingcathodes that consume molecular oxygen, which does not need to be storedwithin the battery (Neburchilov et al., “A review on air cathodes forzinc-air fuel cells” J. Power Sources 2010, 195, 1271-1291). Whilesuccessful as a primary battery in certain commercial applications(e.g., the hearing-aid market), further utility of zinc-air is hinderedby its limited rechargeability, lack of pulse power, and moderateutilization of theoretical discharge capacity (<60%). These limitationsare inherent to the electrochemical behavior of zinc (Zn) in thetraditional anode form-factors that are used in commercial zinc-airbatteries.

When discharging a zinc-air battery containing zinc powder mixed with agelling agent, electrolyte, and binders as the negative electrode, themetallic zinc is oxidized and reacts with the hydroxide ions of theelectrolyte to form soluble zincate ions. The dissolved zincate iondiffuses from its point of electrogeneration until it reachessupersaturation conditions, and rapidly precipitates and dehydrates toform semiconducting zinc oxide (ZnO) (Cai et al.,“Spectroelectrochemical studies on dissolution and passivation of zincelectrodes in alkaline solutions” J. Electrochem. Soc. 1996, 143,2125-2131). Upon electrochemical recharge, the resultant zinc oxide isreduced back to zinc metal, albeit with a shape differing from theinitial discharge. With increasing numbers of discharge-charge cycles,this shape change becomes more pronounced, eventually causing dendritesto grow from the negative electrode until they pierce the separator andcause electrical shorts that end battery operation.

BRIEF SUMMARY

Disclosed herein is an electrochemical cell comprising: an anode currentcollector; an anode in electrical contact with the anode currentcollector; an electrolyte; a cathode current collector; a cathodecomprising nickel, nickel hydroxide, or nickel oxyhydroxide inelectrical contact with the cathode current collector; and a separatorbetween the anode and the cathode. The anode is made by a methodcomprising: providing a mixture comprising a metallic zinc powder and aliquid phase emulsion; drying the mixture to form a sponge; annealingand/or sintering the sponge in an inert atmosphere or under vacuum at atemperature below the melting point of zinc to form an annealed and/orsintered sponge having a metallic zinc surface; and heating the annealedand/or sintered sponge in an oxidizing atmosphere at a temperature abovethe melting point of zinc to form an oxidized sponge comprising a zincoxide shell on the surface of the oxidized sponge. The anode comprises:a continuous network comprising metallic zinc; a continuous network ofvoid space interpenetrating the zinc network; and metallic zinc bridgesconnecting metallic zinc particle cores. The electrolyte fills the voidspace.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention will be readily obtainedby reference to the following Description of the Example Embodiments andthe accompanying drawings.

FIG. 1 shows a photograph (top) and scanning electron micrographs(middle and bottom) of a 3D zinc sponge after heating first in argon andthen in air, showing the fused, through-connected porous network of themonolith and the surface structure of the individual particles withinthe sponge.

FIG. 2 shows a comparison of zinc sponges in the post-heated state(left: A,C,E) and after the electrochemical reduction step (right:B,D,F) as measured by (A,B) electrochemical impedance spectroscopy;(C,D) X-ray diffraction; and (E,F) scanning electron microscopy.

FIG. 3 shows (top) discharge potentials in a half-cell configuration atincreasing applied currents, 5 mA-200 mA for 10-min increments and(bottom) linear dependence of the steady-state discharge voltage withincreasing applied currents.

FIG. 4 shows (top) full-cell zinc-air battery demonstrations, preparedusing the described zinc-sponge anodes and carbon/cryptomelane/Teflon®composite air-cathode and (bottom) discharge profiles of three zinc-aircells at discharge current densities of −5 mA cm⁻², −10 mA cm⁻², and −24mA cm⁻².

FIG. 5 shows (top) diagram of Zn/ZnO sponge symmetrical cell and(bottom) charge-discharge cycling data for up to 45 scans at an imposedload alternating between +24 mA cm⁻² and −24 mA cm⁻².

FIG. 6 shows (top) SEM of a single particle of a fully reduced,all-metal zinc sponge and (bottom) SEM demonstrating the formation of acompact ZnO coating over the surface of the Zn sponge aftercharge/discharge cycling for 45 scans at ±24 mA cm⁻²; note thatmacroscale (>10 μm) dendrites are not observed.

FIG. 7 shows cell configuration of a rechargeable Ni—Zn prototype cellthat comprises the Zn sponge anode.

FIG. 8 shows (top) electrochemical cycling of a Ni—Zn prototype cellcomprising a Zn sponge anode; (middle) expanded subsection of cyclingdata displaying the typical cell voltage as a function of time, and(bottom) expanded subsection of cycling data displaying the typical cellcurrent as a function of time.

FIG. 9 shows scanning electron micrographs of (left) an uncycled Znsponge anode and (right) a Zn sponge anode that has been cycled >85times to 40% DOD_(Zn) (theoretical).

FIG. 10 shows nickel-zinc cells comprising a Zn sponge anode achieves92% of theoretical Zn utilization upon discharge (at 10 mA cm⁻²) and canbe recharged to 95% of initial capacity (at 10 mA cm⁻²).

FIGS. 11A-C show long-term performance of a Ni—Zn single-cell as cycledunder a multi-step duty cycle. FIG. 11A shows the current vs. timeprofile of the duty cycle. FIG. 11B shows the measured cell voltage vs.time curves at (—) early (4,000 cycles) and (---) late (54,000 cycles)points in the 4.5 months-long, non-stop cycling. FIG. 11C showsmicrographic analysis of a post-cycled Zn sponge after ˜54,000 cycles,which verifies that no dendrites are formed.

FIG. 12 shows an illustration exemplifying a mold with a base comprisingtin or a tin-coated metal capable of producing 3D zinc sponge electrodesthat are fused and electronically connected to the tin or a tin-coatedmetal.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the following description, for purposes of explanation and notlimitation, specific details are set forth in order to provide athorough understanding of the present disclosure. However, it will beapparent to one skilled in the art that the present subject matter maybe practiced in other embodiments that depart from these specificdetails. In other instances, detailed descriptions of well-known methodsand devices are omitted so as to not obscure the present disclosure withunnecessary detail.

The fundamental requirements for zinc-containing secondary batteries aretwo-fold. In the case of zinc-air, the air-breathing cathode structuremust contain catalysts for both the oxygen reduction reaction (ORR) forbattery discharge and the oxygen evolution reaction (OER) for thereverse reaction upon recharge. The zinc anode composite requires eitherdendrite-inhibiting additives or its architecture must be designed sothat the current density is uniformly distributed throughout the zincstructure, thereby decreasing the likelihood of dendrite formation andeventual shorting of the battery. This disclosure focuses on thisre-design of zinc architectures to study the application for these zincsponges to be used for secondary zinc-containing battery systems.

Disclosed herein is an approach to replace powdered-bed zinc anodes withhighly porous, monolithic, and 3D through-connected zinc sponges asnegative electrodes for use in current and to-be-developedhigh-performance zinc-containing batteries. In general, the zinc spongesare fabricated by forming a slurry of zinc powder in a two-phase mixturein the presence of emulsion stabilizers to produce highly viscous, yetpourable mixtures that are dried in molds and subsequently thermallytreated to yield robust monolithic electrodes. The zinc sponges mayexhibit high surface areas due to the interconnected pore network (sizedat 10-75 μm), which may lead to an increase in achievable power densityrelative to commercial zinc-containing batteries. After anelectrochemical reduction step, the device-ready electrode isinterconnected in 3D, highly conductive, highly porous, infiltrated withelectrolyte, structurally sound, and provides an ideal platform for usein rechargeable batteries that use zinc anodes or for primary batteriesin which higher utilization of zinc is desired. The fully metallicsponge network provides an electronic environment of improved currentdistribution, thereby inhibiting the formation of dendrites that lead toelectrical shorting. Preliminary characterization of the zinc spongeanode in flooded half-cell configurations is shown, as well asevaluations in full cell zinc-air battery prototypes, and demonstrationof rechargeability without deleterious dendrite formation. US Pat. Appl.Pub. No. 2016/0093890 is incorporated herein by reference, and allmaterials and methods disclosed therein are application to the presentlydisclosed subject matter.

Fully metallic, highly conductive pathways in 3D that allow for improvedcurrent distribution throughout the electrode structure precludes thedischarge-charge cycles from having uneven reaction loci and high localcurrent densities which spur formation of dendrites (Zhang,“Electrochemical thermodynamics and kinetics,” Corrosion andElectrochemistry of Zinc. 1996, 1^(st) Ed.; Arora et al., “Batteryseparators” Chem. Rev. 2004, 104, 4419-4462). In addition, a highlyporous network of zinc allows for confined volume elements with highsurface-to-electrolyte volume with faster concomitant saturation ofzincate upon discharge and more rapid dehydration to ZnO, therebyminimizing shape change.

The electrode contains two bicontinuous interpenetrating networks. Oneis solid and comprises zinc and the other is void space. Thus theelectrode is a porous zinc structure that may be in the form commonlyreferred to as a sponge. The zinc network may contain zinc on both thesurfaces and the interior of the network. That is, it is not zinc coatedonto a non-zinc porous substrate, and it may be pure or nearly pure zincthroughout. The zinc network may also comprise zinc oxide and/or zincoxyhydroxides that form on the surface when the electrode is dischargedin a cell. The zinc network is a fused, monolithic structure in threedimensions. The structure would not be made by merely pressing togetherzinc particles. Such a pressed material would not have the zincparticles fused together as the pressed particles could be separatedfrom each other. The zinc network may have less than 5 wt. % zinc oxideor even less than 1 wt. %. A lower percentage of zinc oxide may resultin better performance of the electrode, however since zinc canspontaneously oxidize in air, it may not be possible for the electrodeto be completely zinc oxide-free.

As used herein, the void space refers to the volume within the structurethat is not the zinc network nor any other material attached thereto.The void space may be filled with a gas or liquid, such as anelectrolyte, and still be referred to as void space.

An example method for making the electrode begins with providing anemulsion of a zinc powder in a liquid phase. Any particle size of thezinc powder may be used, including but not limited to, 100 μm or less.Smaller particle sizes may result in better electrode performance. Theliquid phase of the emulsion may be any liquid or mixtures thereof thatcan be evaporated and in which the zinc powder can be emulsified. Amixture of water and decane is one suitable liquid phase. Theemulsification may be improved by the addition of an emulsifier and/oran emulsion stabilizer. One suitable emulsifier is sodium dodecylsulfate and one suitable emulsion stabilizer is carboxymethylcellulose.Other such suitable emulsifiers and emulsion stabilizers are known inthe art. The zinc metal may be alloyed with indium and bismuth or otherdopants or emulsion additives that suppress gas evolution and corrosionof the sponge, which may improve performance of the electrode. Themixture of zinc and emulsion may have a viscosity of, for example 2-125Pa·s, 2-75 Pa·s, or 50-125 Pa·s.

The emulsion is introduced into a container that defines the desiredsize and shape of the resulting Zn/ZnO monolith, and then dried toremove the liquid component. The dried emulsion yields a porous solidobject comprising Zn/ZnO particles and void, herein designated as a“sponge”; this porous object may be fragile because the zinc powderparticles are not fused together.

The mold may comprise a metal substrate, mesh, or foil (such as tin) oran alloyed substrate, mesh, or foil (such as tin-coated copper) capableof sustaining the temperatures reached in subsequent thermal treatmentsteps. This protocol yields electrodes that are metallically fused toanother metal. Applications for this protocol include, but are notlimited to, electronically connecting the sponge electrode to a currentcollector to be used in an electrochemical cell, such as a battery. Thisprocedure is shown in FIG. 12, where the base of the mold is open and isplaced over a metal mesh (tin-coated copper) with a mesh size throughwhich the zinc particles cannot pass and comprising metals capable ofsustaining the temperatures reached in (bottom) subsequent thermaltreatment steps. The mesh may have an average opening size that is lessthan the d₅₀ particle size of the zinc powder.

Next the sponge is annealed and/or sintered under a low partial pressureof oxygen to form an annealed and/or sintered sponge. Such conditionsmay be found either in an inert atmosphere (examples could include argonor nitrogen flow) or under vacuum, all of which contain a trace amountof oxygen. The annealing and/or sintering is performed at temperaturebelow the melting point of zinc and may be at least two thirds of themelting point of zinc. The temperature ramp may be, for example, 2°C./min, and the dwell time may be for example, at least 30 minutes. Theannealing and/or sintering fuses the zinc particles into a monolithicstructure without causing enough melting to significantly change theoverall morphology. The structure remains a sponge. Any annealing and/orsintering conditions that fuse the zinc particles together may be used.Example conditions include, but are not limited to, annealing and/orsintering in argon at a peak temperature of 200 to 410° C. The fusedstructure contains metallic zinc with interconnecting bridges fusing theparticles together.

Next, the annealed and/or sintered sponge is heated in an oxidizingatmosphere to produce zinc oxide on the surface of a partially oxidizedsponge. The heating is done at a temperature above the melting point ofzinc. This second heating step can improve the strength of the spongefor further handling, such as incorporation into a battery or otherdevice. Since the zinc oxide does not melt and decomposes at a muchhigher temperature than the melting point of zinc, it preserves thesponge structure even at high temperatures. A zinc oxide shell coveringthe fused zinc network is formed, generally preserving the metallic zincbridges and powder particle cores within the shell. Some or all of thebridges may be partially or entirely converted to zinc oxide, but thephysical bridges are not destroyed. Within the zinc oxide shell, themetallic zinc may melt without altering the morphology of the sponge,while potentially further increasing the strength of the structure andthe fusing bridges. Any heating conditions that form the zinc oxide maybe used. Example conditions include, but are not limited to, heating inair at 420 to 650, 700° C., at least greater than the melting point ofthe zinc, or at least 150° C. greater than the melting point of the zincfor at least 30 minutes.

Next, the sponge is returned to the inert atmosphere at a temperature ator above the melting point of the metal, for example 420 to 650, 700°C., at least greater than the melting point of the zinc, or at least150° C. greater than the melting point of the zinc for 30 min or longer.This third heating step can further improve the strength andinterconnectivity of the sponge by terminating additional oxideformation and further fusing the metal core of the sponge backbone,while the shape is protected by the oxide formed on the surfaces in theprevious step. The switch back to an inert atmosphere halts furtherconversion to zinc oxide, to maintain a high amount of zinc in the core.

Optionally, the zinc oxide is then electrochemically reduced back tozinc to form a zinc metal sponge. This sponge contains theinterpenetrating zinc and void space networks. The reduction may takeplace after the electrode is placed in the device, such as a battery,for which it is intended. Any electrochemical conditions that reduce thezinc oxide may be used. It may be done, for example, by applying anegative voltage to the oxidized sponge until the open-circuit potentialvs. zinc is less than 5 mV. As above, the fusing bridges are generallypreserved and converted back to metallic zinc. Some amount of zinc oxidemay be present on the surfaces of the zinc network, as some zinc willoxidize even at room temperature.

This structure differs from one made by a single heating step of heatinga dried emulsion under vacuum at a high temperature above the meltingpoint of zinc. Such heating would rapidly form a shell of zinc oxide oneach individual zinc particle before metallic zinc bridges could form tofuse the particles together. Lacking these bridges, such a structure ishighly fragile and may have less electrical interconnectedness than thepresently disclosed structure.

The final electrode may be used in an electrochemical cell. Such a cellmay comprise an anode current collector, an anode comprising the zincsponge electrode in electrical contact with an anode current collector,an electrolyte filling the void space, a cathode, a cathode currentcollector, and a separator between the anode and the cathode. Theelectrochemical cell may be a zinc-air battery. The construction of suchbatteries is known in the art. When the anode of the cell is fully orpartially discharged, zinc oxide and/or zinc oxyhydroxides may be formedon the surface of the zinc network.

Other possible applications include the use of the zinc sponges in avariety of battery systems containing zinc as the negative electrode.Full-cell batteries (e.g., silver-zinc, nickel-zinc, zinc-carbon,zinc-air, etc.) can be prepared without modification of the fabricationprocedure of the zinc sponges described herein, with or withoutelectrolyte additives. The full-cell batteries described herein mayutilize a nylon screw cap as the cell holder. Alternatives to thisinclude any cell holder that contains zinc as an electrode component.Other alternatives include fabrication procedures designed to yield 3D,through-connected zinc structures, in an effort to produce porous zincmonoliths with uniform current distribution as a means to suppressdendrite formation, enhance cyclability, and/or increase zincutilization in primary or secondary zinc-containing batteries.

In a Ni—Zn cell, the zinc sponge may be infiltrated with a calciumhydroxide solution and dried leaving behind calcium hydroxide on thesurface of the sponge. A suitable electrolyte is a mixture of KOH andLiOH. Any amounts and concentrations of these hydroxides that result ina functional cell may be used.

The development of zinc-containing batteries capable of high-poweroperation and enhanced rechargeability requires a redesign of thearchitecture of the zinc electrode to provide high-surface-areaelectrochemical interfaces and to support improved current distributionto thereby suppress the overgrowth of electrodeposited Zn anddeleterious dendrite formation. Traditional powdered-bed zinc compositespredominantly used as negative electrodes in commercial zinc-containingbatteries (e.g., zinc-air) suffer from low utilization (<60%) oftheoretical specific capacity of Zn, high content of electrolyteadditives, nonuniform current distribution, and limited rechargeability.The electrode disclosed herein describes the preparation of new zincanodes that markedly improve on these drawbacks.

The formation of zinc powder emulsions with subsequent two-stepannealing and/or sintering and electrochemical reduction steps yieldsrobust, scalable, monolithic zinc sponges that are device-ready for avariety of zinc-containing batteries. The resultant zinc spongecomprises two interpenetrating, co-continuous networks of zinc metal andvoid, which improves current distribution throughout the structure ofthe electrode. This feature, inherent to the 3D architecture of theanode, hinders the formation of concentration gradients that wouldotherwise lead to disproportionate reaction centers that encouragegrowth of dendrites that inevitably cause battery shortage. The primaryzinc-air battery used in the example below utilizes >20% more zinc thancommercial powdered-bed zinc anode composites thereby providing higherspecific energy, another feature expected from a metallic network ofimproved current distribution. In addition, a highly porous networkco-continuous with the network of zinc metal allows for confined volumeelements with high ratios of surface-to-electrolyte-volume that promotefaster saturation of zincate upon discharge and more rapid dehydrationto ZnO, thereby minimizing shape change. This notion, coupled with thedendrite suppression resulting from improved current distribution allowsfor rechargeability not seen with the Zn anodes in commercial zinc-airbatteries.

The following examples are given to illustrate specific applications.These specific examples are not intended to limit the scope of thedisclosure in this application.

Example 1

Fabrication of Monolithic Zinc Sponges—

A typical preparation of zinc sponge electrodes begins with theformation of an emulsion of zinc powder in water and decane. In a smallbeaker or scintillation vial, 6.0 g of zinc powder that also contains300-ppm indium and 285-ppm bismuth (as-received from Grillo-Werke AG)was added. The indium and bismuth additives are necessary to raise theoverpotential for hydrogen evolution in alkaline electrolytes, whileeliminating the need for toxic additives such as lead and mercury(Glaeser, U.S. Pat. No. 5,240,793). Water (1.027 mL) and decane (2.282mL) were added along with an emulsifier, sodium dodecyl sulfate (6.3 mg)and emulsion stabilizer, carboxymethylcellulose (0.253 g). The use ofthese ingredients in the formation of zinc emulsions that aresubsequently used to make zinc electrodes has been described previously(Drillet et al., “Development of a Novel Zinc/Air Fuel Cell with a ZnFoam Anode, a PVA/KOH Membrane, and a MnO₂/SiOC-based Air Cathode” ECSTrans. 2010, 28, 13-24). The mixture was stirred rapidly at a rate of1,200 rpm for >15 min to insure complete uptake of zinc into theemulsion. The freely flowing, yet viscous emulsion was poured intocylindrical, polyethylene molds and allowed to air-dry overnight. Themolds used in this example were 1.15-cm in diameter and could yield zincsponges from 1-4 mm thick; however, this procedure is scalable to othersizes and shapes. After 16-24 hours of drying, the mold was inverted torelease the zinc monoliths, which were fragile at this stage. Tostrengthen the zinc sponges, samples were transferred to a tube furnaceand heated under flowing argon at a positive ramp rate of 2° C. min⁻¹ toan annealing temperature of 400° C. and held for 2 h. The argon flow wasthen removed and the tube was opened to ambient air and heated for asecond step at 2° C. min⁻¹ to 650° C. and held for 2 h. This final stepencapsulates the surface of the annealed zinc particles with a shell ofZnO needles, which is necessary to impart additional strengtheningcharacter to the zinc sponges. After 2 h, the tube was allowed to coolwithout any rate control. The resultant monolith was characterized withscanning electron microscopy (SEM) as seen in FIG. 1.

Example 2

Zinc Sponges as a Negative Electrode in Zinc-Air Batteries—

The intentional introduction of oxide to the zinc sponges enhances themechanical integrity, allowing them to be routinely handled withdecreased risk of fracture; however, a layer of oxide on the zinc lowersthe initial capacity upon discharge and introduces contact resistancewhen assembling the zinc-containing cell (see Nyquist plots fromelectrochemical impedance spectroscopy (EIS) in FIG. 2A). In order toelectrochemically reduce the ZnO coating, the sponge is used as part ofthe working electrode in a half-cell, three-electrode configuration witha Pt counter electrode and zinc quasi-reference electrode in 6 M KOH. Azinc sponge was placed in an envelope of tin-coated-copper mesh to formthe working electrode. (Tin contacts were used because tin isgalvanically compatible with zinc and corrosion of the electrode issuppressed that would otherwise be rampant with other current collectors(e.g., nickel, copper, etc.).) In a typical experimental sequence, theopen-circuit potential (OCP) of the cell was measured versus a metalliczinc quasi-reference electrode, and then an initial EIS measurement wasperformed. The initial OCP typically exceeded 40 mV vs. Zn, and the realimpedance (R_(CT)) was much higher than that expected for metalliccontact, consistent with the presence of a poorly conducting zinc oxidecoating on the zinc sponge. Electrochemical reduction of theoxide-coated sponge to its metallic zinc counterpart was achieved byapplying a constant potential of −50 mV for 30 min, followed byadditional EIS and OCP measurements. This sequence was repeated untilthe open-circuit potential was stable at or near 0 mV, indicatingcomplete reduction to zinc metal. The charge-transfer resistancedecreased to less than 0.2 Ωcm⁻² for the electroreduced zinc sponge, ascompared to the post-heated zinc sponge, where resistance exceeded 60Ωcm⁻² (FIGS. 2A,B). Conversion of ZnO to Zn metal was confirmed usingX-ray diffraction (Rigaku, FIGS. 2C, 2D), which showed a loss of the ZnOreflections, leaving only metallic zinc after electroreduction. Inaddition, after reduction there was no obvious loss in the porosity ormechanical strength of the zinc monoliths; however, a measurable massloss was recorded, due to oxygen mass lost as ZnO is reduced to Zn aswell as some corrosion of zinc to form soluble products that are lost tothe alkaline electrolyte. Based on twelve control experiments, using Znsponges annealed and oxidized using the thermal/atmosphere treatmentdescribed above, the average mass loss associated with thiselectrochemical reduction step is 23.9±3.4%. FIGS. 2E, 2F highlight thechanging morphology after the reduction step, effectively removing theshell of zinc oxide associated with the as-prepared sponges.

The reduction step described above successfully lowered the amount ofzinc oxide present in the sponge, which would otherwise limit capacityand increase resistance when incorporated into a zinc-containingbattery. The half-cell testing configuration provides a quality check onthe impedance characteristics of the anode prior to use in full-cellbatteries, but it can also be a useful tool to study the powercapabilities of zinc sponges as anodes. For example, a zinc monolithicsponge (1.15-cm diameter; 3.5-mm thick) was attached to a tin currentcollector with LOCTITE® Hysol® 1C™ epoxy completely surrounding allcomponents except for the face of the zinc sponge. A reducing voltage of−50 mV versus zinc was applied for 50 min followed by discharging (i.e.,oxidizing) the zinc sponge at constant current (5 mA) for 10 min tomeasure the steady-state discharge voltage. This protocol was repeatedwith increasing applied currents as shown in FIG. 3. The galvanostaticexperiments revealed a linear dependence of the steady-state dischargevoltage on applied current. Even at an imposed current of 200 mA (193 mAcm⁻²), the overpotential required for maintaining this current densitywas only 230 mV. The ability to maintain low overpotentials even at highload (current density) is an enabling characteristic of the zinc spongearchitecture. Conventional zinc-containing batteries, includingzinc-air, typically operate with up to a 500 mV drop with respect toopen-circuit voltage (Linden, “Zinc/air cells” Handbook of Batteries.1984, 2^(nd) Ed.).

Once the zinc sponge was fully reduced to Zn⁰, it was ready to be usedas a negative electrode in a full-cell battery. The prototype zinc-aircell used for preliminary testing is based on a 1.8-cm nylon screw cover(Hillman Group), which snaps together with a 6-mm hole on the top face,which serves as the air-breathing side of the cell. A platinum wire wasattached to a tin current collector and used as the negative terminalduring battery testing. Following the separate electrochemical reductionstep, the zinc sponge, still infiltrated with a 6 M KOH, was submergedin a gel electrolyte synthesized from 6 g of polyacrylic acid dissolvedin 100 mL of 6 M KOH. To prepare the full Zn-air cell, excess gel wasdabbed away from the gel-dipped zinc sponge, leaving only a thincoating. The viscous gel electrolyte insures the zinc sponge remainsfully infiltrated with liquid electrolyte while slowing the evaporationof solvent. The zinc sponge was placed on the tin current collector, andthen topped with an aqueous-compatible separator with dimensionsslightly larger than the diameter of the zinc sponge (1.15 cm). Thepositive electrode terminal comprises an air-cathode composite of Ketjenblack carbon, cryptomelane, and Teflon® attached to a piece of nickelmesh, attached to a platinum wire lead. The results of typical zinc-airfull cells utilizing these zinc sponge anodes are shown in FIG. 4. Inthese examples, all initial OCPs were measured above 1.4 V prior todischarging the full cells at −5.0, −10, and −24 mA cm⁻². The averagedischarge voltage for these cells was 1.25, 1.19, and 1.13 V,respectively, each with a cutoff voltage of 0.9 V. The correspondingspecific capacity obtained at the −5, −10, and −24 mA cm⁻² dischargeswere 728, 682, and 709 mAh g_(Zn) ⁻¹ with respective specific energydensities of 907, 834, and 816 Wh kg_(Zn) ⁻¹ and corresponding zincutilization for these cells of 89%, 83%, and 86%. These metrics are animprovement over standard commercial zinc-powder composite anodes, whichgenerally only utilize 50-60% of the theoretical specific capacity ofzinc (Zhang, “Fibrous zinc anodes for high power batteries.” J. PowerSources. 2006, 163, 591).

Example 2

Reversibility of Zinc Sponge Anodes—

In order to study the reversibility of the 3D zinc sponge in a batteryconfiguration, without the requirement of having an optimized cathode(e.g., bifunctionally catalytic for the ORR or OER), a symmetricalelectrochemical cell containing an all-metal Zn sponge versus a Zn/ZnOsponge separated by an aqueous-compatible separator was used. The Zn/ZnOsponge was prepared by electroreducing some of the ZnO present from thepost-heated sponge by applying −50 mV versus zinc for 10-min increments.The EIS and the OCP were measured after each cycle. The reduction ofthis sponge was terminated when the R_(CT) in the EIS fell below 0.5Ωcm⁻², but the OCP remained greater than 30 mV vs. Zn, indicating highconductivity throughout the sponge network, yet ZnO remained. A secondpost-heated sponge was reduced at −50 mV versus Zn for 30-min incrementsas described in [0032] until it was fully reduced to an all metal Znsponge with an OCP very close to 0 mV vs. Zn. For the construction ofthe symmetrical cell, the negative electrode was an all-metal Zn spongein electrical contact with a tin foil current collector and the positiveelectrode was the Zn/ZnO sponge electrode, also in contact with a tinfoil current collector. Both sponge electrodes were pre-infiltrated with6 M KOH and were separated by an aqueous-compatible separator (see FIG.5). For the first step in the evaluation of the symmetrical cell, −24 mAcm⁻² was applied for 1 h to reduce some of the ZnO in the Zn/ZnO sponge,which is coupled to the oxidation of Zn in the opposing sponge. Then +24mA cm⁻² was applied in the second step to initiate the reversereactions. The full symmetrical cell was cycled at ±24 mA cm⁻² until oneof the steps crossed the ±100 mV threshold. No electrical shorts wereobserved. For this example, the symmetrical cell was cycled for 45charge-discharge cycles at a depth-of-discharge of ˜23%. Forpost-cycling analysis, the electrodes were removed from the cell, rinsedthoroughly, and dried in vacuo overnight. Scanning electron micrographsshow that the sponges maintained their porosity post-cycling (FIG. 6).In addition, no obvious signs of shape change, dendrite formation, ornonuniform deposition were observed. The cycling of the zinc spongesresulted in a compact layer of zinc or zinc oxide on the surface of theparticles of the monolith rather than flowery dendrites, demonstratingan increase in cyclability that is a result of this well-wired,well-plumbed zinc sponge architecture.

Example 3

Zinc Sponges as a Negative Electrode in Rechargeable Nickel-ZincBatteries—

Following the electrochemical reduction step to convert a thermallytreated Zn sponge to the metallic Zn⁰ analog, the Zn sponge wasthoroughly rinsed with deionized water, dried, and re-weighed in orderto get an accurate assessment of Zn active material capacity beforeprototype testing. The total mass of the ˜1-cm² reduced Zn sponge usedfor this example was 0.140 g. Once dried overnight, the Zn sponge wasvacuum-infiltrated with a water-based slurry of Ca(OH)₂ (in this case0.4 g mL_(H) ₂ _(O) ⁻¹), which is known to aid Zn anode rechargeability(U.S. Pat. No. 5,863,676). The Ca(OH)₂-infused Zn sponge was removedfrom the Ca(OH)₂ slurry and gently rinsed with water to remove excessCa(OH)₂, dried overnight in vacuo, and re-weighed to yield a Zn spongeinfused with, as an example, ˜6 wt. % Ca(OH)₂. The dried electrode wasthen infiltrated with the electrolyte that comprises 6 M KOH and 1 MLiOH, the latter component known to enhance the cyclability ofNiOOH/Ni(OH)₂ electrodes in Ni—Zn batteries (U.S. Pat. No. 4,224,391; WO2003034531 A1). Once infiltrated with electrolyte, the Zn sponge wasplaced atop a tin flag current collector, and then topped with two typesof separators: microporous (e.g., Celgard 3501) and nonwoven (e.g.,Freudenberg 700/28K). The nonwoven separator was pre-infiltrated withthe 6 M KOH+1 M LiOH electrolyte. The positive electrode terminalcomprised a harvested, commercial nickel oxyhydroxide (NiOOH) cathode(recovered from a charged AA Ni—Zn cell) in mechanical contact with apiece of Ni mesh and a platinum wire. A second nonwoven separator thatwas infiltrated with electrolyte was placed on top of the cathodeassembly. All cell materials were housed in a 1.8-cm nylon screw cover(Hillman Group). An expanded view of the cell assembly is shown in FIG.7.

In this example, the open-circuit voltage was 1.88 V prior to cycling.Electrochemical cycling commenced with an initial break-in discharge of5 mA (˜5 mA cm⁻²) for 57 mAh (409 mAh g_(Zn) ⁻¹), equivalent to 50%depth-of-discharge with respect to the total amount of theoretical Znactive material present. Subsequent cycles used a charging rate of 10 mA(˜10 mA cm⁻², a current density that has been shown previously to launchcell-shorting dendrites) and a discharge rate of 25 mA (˜25 mA cm⁻²),both with terminal capacities of 46 mAh (328 mAh g_(Zn) ⁻¹), equivalentto 40% depth-of-discharge with respect to the total amount oftheoretical Zn active material present. A two-hour potentiostatic chargeat 1.93 V was added to the end of each constant-current charging step toensure complete conversion of NiOOH to Ni(OH)₂. Cycling tests wereterminated after the discharge voltage fell below 1.3 V, indicating thatthe discharge capacity no longer reached the desired capacity level. TheNi—Zn prototype cell maintained 100% of the desired capacity for 85cycles before capacity fade was observed. No electrical shorts wereobserved. The results of the rechargeable Ni—Zn full cell prototype testutilizing a Zn sponge anode are shown in FIG. 8. After cycling, thestill-monolithic Zn sponges were removed from the cell, gently washedwith deionized water, and imaged using scanning electron microscopy. Themicrographs reveal that the Zn sponges retain their porosity andinterconnectedness, with no evidence of macroscale dendrites (FIG. 9).The onset of cell polarization and subsequent cycling termination isascribed to a combination of phenomena unrelated to the Zn sponge anode,such as gas evolution, current collector corrosion, and/or solventevaporation which arises from using a non-hermetically sealed cell. Notethat capacity fade has been reversed in analogous Ni—Zn cells that useZn sponge anodes by pipetting electrolyte and/or water to the fadingcells.

Example 4

Zinc Sponges as a Negative Electrode in High Capacity RechargeableNickel-Zinc Batteries—

Following the electrochemical reduction step to convert a thermallytreated Zn sponge to the metallic Zn⁰ analog, the Zn sponge wasthoroughly rinsed with deionized water, dried, and re-weighed in orderto get an accurate assessment of Zn active material capacity beforeprototype testing. The total mass of the ˜1-cm⁻² reduced Zn sponge usedfor this example was 0.111 g and was infiltrated with Ca(OH)₂ asdescribed above. Once infiltrated with the 6 M KOH+1 M LiOH electrolyte,the Ni—Zn cell was constructed according to the previous example.

The higher cell voltage of Ni—Zn over traditional, single-use alkalinebatteries (MnO₂—Zn) is a compelling feature if it can be coupled toessentially complete utilization of the Zn anode. The ability of Znsponge anodes to discharge to high Zn mass-normalized capacity and berecharged without inducing dendritic shorts was probed by exhaustivelydischarging Ni—Zn cells at a current density of −10 mA cm⁻² (C/9) andthen recharging at the same rate (FIG. 10). This cell reached 92%DOD_(Zn) (753 mA h g_(Zn) ⁻¹; 1226 Wh kg_(Zn) ⁻¹) with an averagedischarge voltage of 1.62 V and could be recharged to 95% capacity fromthese extreme depths.

Example 5

Zinc Sponges as a Negative Electrode in Rechargeable Nickel-ZincBatteries with Complex Duty Cycles—

Following the electrochemical reduction step to convert a thermallytreated Zn sponge to the metallic Zn⁰ analog, the Zn sponge wasthoroughly rinsed with deionized water, dried, and re-weighed in orderto get an accurate assessment of Zn active material capacity beforeprototype testing. The total mass of the ˜1-cm² reduced Zn sponge usedfor this example was 0.112 g and was infiltrated with Ca(OH)₂ asdescribed above. Once infiltrated with the 6 M KOH+1 M LiOH electrolyte,the Ni—Zn cell was constructed according to the previous example.

The prototype Ni—Zn cell for this example was prepared to demonstratethe capabilities of the Zn sponge to undergo many thousands of lowdepth-of-discharge duty cycles (<1% DOD_(Zn)), as would be relevant to,for example, start-stop batteries in micro-hybrid vehicles. Duty cyclesfor start-stop batteries, for example, include pulses for start andrestart as well as longer duration constant-use loads for auxiliaryapplications. In this example, the open-circuit voltage was measured at1.85 V and then pre-discharged to 20% DOD_(Zn) at ˜5 mA cm⁻² prior tocycling the Ni—Zn cells according to the duty cycle of FIG. 11A. Thechoice of the duration and current density per step has been detailedpreviously (Parker, et al., Science, Vol. 356, pp. 415-418). Greaterthan 50,000 cycles (FIG. 11B) were achieved with cycling stopped onlywhen the high load pulse (˜65 mA cm⁻²) reached a pre-set voltage limitof 0.8 V. No electrical shorts were observed. Note that the cumulativedischarge capacity for ˜54,000 cycles is ˜3× that achieved in the 40%DOD_(Zn)/85+ cycles discussed above. Postmortem analysis of themonths-long-cycled (still non-hermetically sealed) cells revealed a drycell concomitant with an increased charge-transfer resistance. Thepost-cycled Zn sponge remains visibly monolithic; scanning electronmicroscopy reveals that the pore-solid architecture of the Zn sponge isretained and no anomalous macroscale dendrites are observed (FIG. 11C).

Obviously, many modifications and variations are possible in light ofthe above teachings. It is therefore to be understood that the claimedsubject matter may be practiced otherwise than as specificallydescribed. Any reference to claim elements in the singular, e.g., usingthe articles “a,” “an,” “the,” or “said” is not construed as limitingthe element to the singular.

What is claimed is:
 1. An electrochemical cell comprising: an anodecurrent collector; an anode in electrical contact with the anode currentcollector; wherein the anode is made by a method comprising: providing amixture comprising a metallic zinc powder and a liquid phase emulsion;drying the mixture to form a sponge; annealing and/or sintering thesponge in an inert atmosphere or under vacuum at a temperature below themelting point of zinc to form an annealed sponge having a metallic zincsurface; and heating the annealed sponge in an oxidizing atmosphere at atemperature above the melting point of zinc to form an oxidized spongecomprising a zinc oxide shell on the surface of the oxidized sponge;wherein the anode comprises: a continuous network comprising metalliczinc; a continuous network of void space interpenetrating the zincnetwork; and metallic zinc bridges connecting metallic zinc particlecores; an electrolyte filling the void space; a cathode currentcollector; a cathode comprising nickel, nickel hydroxide, or nickeloxyhydroxide in electrical contact with the cathode current collector;and a separator between the anode and the cathode.
 2. Theelectrochemical cell of claim 1, wherein the surface of the zinc networkcomprises one or more of zinc oxide and zinc oxide oxyhydroxides.
 3. Theelectrochemical cell of claim 1, wherein the zinc powder or liquid phasecomprises an additive that suppresses gas evolution and corrosion of thesponge.
 4. The electrochemical cell of claim 3, wherein the additivecomprises bismuth and indium.
 5. The electrochemical cell of claim 1,wherein the method of making the anode further comprises: heating theoxidized sponge in an inert atmosphere at a temperature above themelting point of the zinc.
 6. The electrochemical cell of claim 1,wherein the surface of the anode comprises calcium hydroxide.
 7. Theelectrochemical cell of claim 1, wherein the electrolyte comprisespotassium hydroxide and lithium hydroxide.
 8. An electrochemical cellcomprising: an anode current collector; an anode in electrical contactwith the anode current collector; wherein the anode is made by a methodcomprising: providing a mixture comprising a metallic zinc powder and aliquid phase emulsion; drying the mixture to form a sponge; annealingand/or sintering the sponge in an inert atmosphere or under vacuum at atemperature below the melting point of zinc to form an annealed spongehaving a metallic zinc surface; heating the annealed sponge in anoxidizing atmosphere at a temperature above the melting point of zinc toform an oxidized sponge comprising a zinc oxide shell on the surface ofthe oxidized sponge; and electrochemically reducing the zinc oxide toform a zinc metal sponge; wherein the anode comprises: less than 5 wt. %of zinc oxide; a continuous network comprising metallic zinc; acontinuous network of void space interpenetrating the zinc network; andmetallic zinc bridges connecting metallic zinc particle cores; anelectrolyte filling the void space; a cathode current collector; acathode comprising nickel, nickel hydroxide, or nickel oxyhydroxide inelectrical contact with the cathode current collector; and a separatorbetween the anode and the cathode.